module-info.class spec and attributes
David M. Lloyd
david.lloyd at redhat.com
Tue Jun 28 20:43:15 PDT 2011
Yeah I agree, following the mold of classes and package-info is a big
mistake here. Not sure if it's simple cargo-cultism or what.
In JBoss Modules we actually use XML, and performance is very good. If
you used a dedicated text format I'm sure that performance would be
quite good enough at runtime.
On 06/28/2011 04:24 PM, David Bosschaert wrote:
> I wonder why such a binary .class file for module declarations is
> really needed. Why not go for a textual file, for instance
> META-INF/module-info.jmod (or something like that) and declare the
> module declarations in there using a textual format of some sort?
>
> Like the other files in META-INF, it will become part of the ultimate
> jar as-is so compilation into a .class file is not needed nor are
> extensions to the .class file specification.
>
> IMO there are multiple benefits associated keeping the textual format.
> People using the jar can use a simple zip tool to look inside the jar
> to see what its dependencies are. Additionally tools can easily be
> written to read the module-info.jmod file without needing to resort to
> classfile parsing libraries. The module information can be of use to
> both developer as well as deployer tools.
>
> Finally it's probably easier to make the textual file extensible, as
> required by [1], without resorting to inefficient reflection-style
> operations.
>
> Best regards,
>
> David
>
> [1] http://openjdk.java.net/projects/jigsaw/doc/draft-java-module-system-requirements-12#extensible-module-declarations
>
> On 25 June 2011 02:22, Jonathan Gibbons<jonathan.gibbons at oracle.com> wrote:
>> Minor not-cut-n-paste typo:
>>
>>> The ModuleData attribute
>>>
>>> ModuleExtension_attribute {
>>> u2 attribute_name_index;
>>> u4 attribute_length;
>>> u2 data_index;
>>> }
>>
>> -- Jon
>>
>>
>>
>> On 06/24/2011 06:01 PM, Alex Buckley wrote:
>>>
>>> Since attachments seem to be scrubbed, here is the text of the proposal,
>>> obviously without links to the requirements document.
>>>
>>>
>>> ClassFile changes for Java Modules
>>>
>>> Binary form of a module declaration (a.k.a. module-info.class)
>>> A compilation unit that contains a module declaration (and potentially
>>> additional arbitrary content) is compiled to a ClassFile structure like any
>>> other compilation unit.
>>>
>>> By convention, the name of a compilation unit that contains a module
>>> declaration is module-info.java, echoing the package-info.java convention
>>> for a compilation unit that contains solely a package declaration.
>>> Consequently, by convention, the name for the compiled form of a module
>>> declaration is module-info.class.
>>>
>>> A new flag in the ClassFile.access_flags item, ACC_MODULE (0x0800),
>>> indicates that the ClassFile represents a module rather than a class or
>>> interface. Note that this flag plays a similar role to ACC_INTERFACE
>>> (0x0200), and does not describe accessibility of a class or interface.
>>>
>>> If ACC_MODULE is set in ClassFile.access_flags, then no other flag in
>>> ClassFile.access_flags may be set, and the following rules apply to the rest
>>> of the ClassFile structure:
>>>
>>> * major_version, minor_version:>=52.0 (Java SE 8 and above)
>>> * this_class: [Module's name in internal form (JVMS 4.2.1)]/module-info
>>>
>>> (Traditionally, if this_class indicates "P/Q/R", then the ClassFile
>>> can be expected to live in a file R.class in a directory P/Q representing a
>>> package. This explains why "/module-info" is a suffix in this_class above:
>>> if this_class indicates "P/Q/module-info", then the ClassFile can be
>>> expected to live in a file module-info.class in a directory P/Q representing
>>> a module. The "real name" of the module, shorn of "/module-info", can be
>>> obtained from the Module attribute.)
>>> * super_class, interfaces_count, fields_count, methods_count: 0
>>> * attributes: One Module attribute must be present, to record the name
>>> and version of the module. At most one of each of the ModuleRequires,
>>> ModuleProvides, ModulePermits, ModuleExports, ModuleClass, and ModuleData
>>> attributes may be present. Except for these attributes and Synthetic,
>>> SourceFile, SourceDebugExtension, and Deprecated, none of the pre-defined
>>> attributes in JVMS 4.7 may appear.
>>>
>>> The CONSTANT_ModuleId_info structure
>>>
>>> The CONSTANT_ModuleId_info structure in the constant pool is used to
>>> represent a pair of a module name and a module version.
>>>
>>> CONSTANT_ModuleId_info {
>>> u1 tag;
>>> // 19
>>> u2 name_index;
>>> // Points to a CONSTANT_Utf8_info representing module name in internal
>>> form (JVMS 4.2.1)
>>> u2 version_index;
>>> // Optionally points to a CONSTANT_Utf8_info representing module
>>> version
>>> }
>>>
>>> If the version_index item is 0 (as opposed to pointing to a
>>> CONSTANT_Utf8_info structure which holds the string "0"), then no module
>>> version is present.
>>>
>>> If the version_index item is not 0, then a module version is present.
>>> There is no required structure or semantics for the underlying UTF8 string
>>> which represents a module version.
>>>
>>> The Module attribute
>>>
>>> Module_attribute {
>>> u2 attribute_name_index;
>>> u4 attribute_length;
>>> u2 module_id_index;
>>> }
>>>
>>> The items of the Module_attribute structure are as follows:
>>>
>>> attribute_name_index
>>> The value of the attribute_name_index item must be a valid index into
>>> the constant_pool table. The constant_pool entry at that index must be a
>>> CONSTANT_Utf8_info structure representing the string "Module".
>>> attribute_length
>>> The value of the attribute_length item must be 2.
>>> module_id_index
>>> The value of the module_id_index item must be a valid index into the
>>> constant_pool table. The constant_pool entry at that index must be a
>>> CONSTANT_ModuleId_info structure representing the name and version of the
>>> module represented by this ClassFile.
>>> The name must be equal to the module name indicated by the
>>> ClassFile.this_class item without the "/module-info" suffix.
>>>
>>> The ModuleRequires attribute
>>>
>>> ModuleRequires_attribute {
>>> u2 attribute_name_index;
>>> u4 attribute_length;
>>> u2 requires_length;
>>> { u2 requires_index; u1 flags; } requires_table[requires_length];
>>> }
>>>
>>> attribute_name_index
>>> The value of the attribute_name_index item must be a valid index into
>>> the constant_pool table. The constant_pool entry at that index must be a
>>> CONSTANT_Utf8_info structure representing the string "ModuleRequires".
>>> attribute_length
>>> The value of the attribute_length item is the length of the attribute
>>> excluding the initial six bytes.
>>> requires_length
>>> The value of the requires_length indicates the number of entries in the
>>> requires_table.
>>> requires_table
>>> Each requires_index must be a valid index into the constant_pool table.
>>> The constant_pool entry at that index must be a CONSTANT_ModuleId_info
>>> structure representing the target module on which this module depends.
>>> The value of the associated flags item is as follows:
>>>
>>> * 0x01 indicates this dependency on the target module is optional.
>>> * 0x02 indictates the target module's types must be loaded by the
>>> same defining classloader as the types of the module represented by this
>>> ClassFile.
>>>
>>> A module name may be referenced by at most one entry in a
>>> requires_table.
>>>
>>> The ModulePermits attribute
>>>
>>> ModulePermits_attribute {
>>> u2 attribute_name_index;
>>> u4 attribute_length;
>>> u2 permits_length;
>>> { u2 permits_index } permits_table[permits_length];
>>> }
>>>
>>> attribute_name_index
>>> The value of the attribute_name_index item must be a valid index into
>>> the constant_pool table. The constant_pool entry at that index must be a
>>> CONSTANT_Utf8_info structure representing the string "ModulePermits".
>>> attribute_length
>>> The value of the attribute_length item is the length of the attribute
>>> excluding the initial six bytes.
>>> permits_length
>>> The value of the permits_length indicates the number of entries in the
>>> permits_table.
>>> permits_table
>>> The value of each permits_index in this item must be a valid index into
>>> the constant_pool table. The constant_pool entry at that index must be a
>>> CONSTANT_ModuleId_info structure representing a module which is permitted to
>>> have a dependency on the module represented by this ClassFile.
>>> (It is possible that a module system uses only the name of the
>>> permitted module, not its version, in determining visibility.)
>>> A module name may be referenced by at most one entry in a
>>> permits_table.
>>>
>>> The ModuleProvides attribute
>>>
>>> ModuleProvides_attribute {
>>> u2 attribute_name_index;
>>> u4 attribute_length;
>>> u2 provides_length;
>>> { u2 provides_index; } provides_table[provides_length];
>>> }
>>>
>>> attribute_name_index
>>> The value of the attribute_name_index item must be a valid index into
>>> the constant_pool table. The constant_pool entry at that index must be a
>>> CONSTANT_Utf8_info structure representing the string "ModuleProvides".
>>> attribute_length
>>> The value of the attribute_length item is the length of the attribute
>>> excluding the initial six bytes.
>>> provides_length
>>> The value of the provides_length indicates the number of entries in the
>>> provides_table.
>>> provides_table
>>> The value of each provides_index in this item must be a valid index
>>> into the constant_pool table. The constant_pool entry at that index must be
>>> a CONSTANT_ModuleId_info structure representing a module that is an alias
>>> for the module represented by this ClassFile.
>>> A module name may be referenced by at most one entry in a
>>> provides_table.
>>>
>>> The ModuleExports attribute
>>>
>>> ModuleExports_attribute {
>>> u2 attribute_name_index;
>>> u4 attribute_length;
>>> u2 exports_length;
>>> { u2 exports_index; u1 export_kind; u2 source_index; }
>>> exports_table[exports_length];
>>> }
>>>
>>> attribute_name_index
>>> The value of the attribute_name_index item must be a valid index into
>>> the constant_pool table. The constant_pool entry at that index must be a
>>> CONSTANT_Utf8_info structure representing the string "ModuleExports".
>>> attribute_length
>>> The value of the attribute_length item is the length of the attribute
>>> excluding the initial six bytes.
>>> exports_length
>>> The value of the exports_length indicates the number of entries in the
>>> exports_table.
>>> exports_table
>>> The value of each export_index in this item must be a valid index into
>>> the constant_pool table. The constant_pool entry at that index must be a
>>> CONSTANT_Utf8_info structure representing an entity to be exported by the
>>> module represented by this ClassFile.
>>> The value of the associated export_kind item indicates the kind of
>>> entity to be exported, as follows:
>>>
>>> * 0x01 indicates the entity is a class or interface; it is the
>>> entity to be exported.
>>> * 0x02 indicates the entity is a class or interface; it and its
>>> member types are the entities to be exported.
>>> * 0x04 indicates the entity is a package; the member top-level
>>> classes and interfaces (and their member types) of this package are the
>>> entities to be exported.
>>> * 0x08 indicates the entity is a package; the member top-level
>>> classes and interfaces (and their member types) of this package and all its
>>> subpackages (recursively) are the entities to be exported.
>>>
>>> The value of the associated source_index item must be a valid index
>>> into the constant_pool table where the entry is a CONSTANT_ModuleId_info
>>> structure representing the module which declared the exported entity. (In
>>> most cases, where the exported entity is declared in the current module,
>>> source_index will simply point to the same constant pool entry as the Module
>>> attribute. But where the exported entity is re-exported, source_index may
>>> assist resolution and diagnostics.)
>>>
>>> The ModuleClass attribute
>>>
>>> ModuleClass_attribute {
>>> u2 attribute_name_index;
>>> u4 attribute_length;
>>> u2 main_class_index;
>>> }
>>>
>>> attribute_name_index
>>> The value of the attribute_name_index item must be a valid index into
>>> the constant_pool table. The constant_pool entry at that index must be a
>>> CONSTANT_Utf8_info structure representing the string "ModuleClass".
>>> attribute_length
>>> The value of the attribute_length item is 2.
>>> main_class_index
>>> The value of the main_class_index item must be a valid index into the
>>> constant_pool table. The constant_pool entry at that index must be a
>>> CONSTANT_Class_info structure representing the name of the class which is
>>> the entrypoint to the module represented by this ClassFile.
>>>
>>> The ModuleData attribute
>>>
>>> ModuleExtension_attribute {
>>> u2 attribute_name_index;
>>> u4 attribute_length;
>>> u2 data_index;
>>> }
>>>
>>> attribute_name_index
>>> The value of the attribute_name_index item must be a valid index into
>>> the constant_pool table. The constant_pool entry at that index must be a
>>> CONSTANT_Utf8_info structure representing the string "ModuleData".
>>> attribute_length
>>> The value of the attribute_length item is 2.
>>> data_index
>>> The value of the main_class item must be a valid index into the
>>> constant_pool table. The constant_pool entry at that index must be a
>>> CONSTANT_Utf8_info structure representing the content, other than the module
>>> declaration, of the compilation unit that declared the module represented by
>>> this ClassFile.
>>
>>
--
- DML
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